1. Field of Invention
This invention pertains to telecommunications systems, and particularly to the routing of calls through a telecommunications system for a relocated subscriber and location of any telecommunications system resources.
2. Related Art and Other Considerations
A telecommunications network typically includes a number of physical nodes, often referred to as local exchanges, to which subscribers are connected. The local exchanges are generally connected in the telecommunications network by other physical nodes, known as transit exchanges.
To simplify the routing of calls through the network and to have a good structure of a telephone numbering plan, each local exchange is allocated one or more unique exchange number groups. The telephone number of a subscriber typically includes both an exchange number group (typically a 10,000 number block) for the exchange to which a subscriber is connected, and a number in that group which is peculiar to the subscriber. For example, a subscriber having a telephone number "881-1657" is connected to a local exchange having an exchange number group "881", and within that group the subscriber has a subscriber number of "1657". The subscriber's telephone number is published or otherwise circulated as his directory telephone number e.g., in a telephone directory or book.
A common way of routing a call through a telecommunications network to a final destination is to use the directory telephone number of the called party (e.g., the called subscriber). In particular, the called party's directory telephone number occupies an address signal field of an ISUP parameter known as the "Called Party Number" parameter ("CdPN"), with the "Called Party Number" parameter ("CdPN") being a routing or address message utilized for routing purposes.
Using the directory telephone number of the called party for routing purposes has numerous problems, particularly when a subscriber physically relocates. If the relocating subscriber wants to keep his original directory number, he must be connected by a special physical line to his same local exchange. Such physical connection is quite expensive when the subscriber has moved far away from the local exchange. If the relocation is such that the subscriber's connection to the telecommunications network changes from an old local exchange to a new local exchange, on the other hand, the provider of the telecommunications network is not able to accommodate relocation of the subscriber without changing the subscriber's directory telephone number.
Changing a directory telephone number in the event of relocation of a subscriber occasions expense and effort for both the subscriber and the telecommunications provider. For the provider, it is expensive to administer the changes of directory telephone numbers when a subscriber relocates from one area to another. The administration required by the provider includes both efforts to define available new numbers in the new location (i.e., at the new local exchange) and to update the published directory. The relocated subscriber incurs expense in providing notice of the new directory number to potential callers (friends and business contacts). If such notice is not provided or retained by the potential callers, calls may not be placed to the relocated subscriber. Loss of calls to a relocated subscriber can result in loss of social or business opportunity.
Various mollifying measures are currently available to a relocated subscriber. One measure is to provide the caller who dials an old directory number with a audible message announcing the new directory number of the relocated subscriber, which new directory number must be noted and dialed by the caller. Another measure is automatic forwarding of the call from the old directory number to the new directory number. Yet another measure is a mechanical reconnection of the subscriber line to the new local exchange for the relocated subscriber. In a further measure, known as "drop back", a terminating exchange, upon receiving a call and detecting that its former subscriber has moved, sends a "release" message containing the number to which the call should be forwarded.
None of the remedial measures described above are entirely satisfactory. The audible message and automatic forwarding are typically only temporary services. Moreover, these services tax network resources. For the audible message, switching resources are utilized in the local exchange to which the relocated subscriber was formerly connected. In the case of automatic call forwarding, network signalling and hardware resources are required, leading to loss of capacity and revenue for the network provider. Mechanical reconnection may solve the problem in a small area, but is very expensive if not impossible to implement on a large scale. Moreover, hardware in the exchanges is not designed or intended for frequent physical reconnections of subscriber lines. The "drop back" approach is precarious when different service providers are involved in the relocation, and may spawn curious side effects if the subscriber changes service or access types.
A station number portability arrangement for a private network is described in U.S. Pat. No. 4,754,479 to Bicknell et al. The Bicknell arrangement allows a subscriber who is ported from an original switch to a new switch to retain an originally assigned number regardless of any numbering plan constraints. All switches of a relocateablity cluster contain a common database which identifies the present location of subscribers via a switch identity. The common data base must be provided at each switching node.
Bicknell-type databases at each physical node of a public telecommunications system would be formidable to implement and maintain. Consider, for example, a public telecommunications network covering an entire country and including hundreds or even thousands of exchanges. How difficult it would be to update the databases at each exchange when a subscriber relocates. Moreover, the memory requirements exacted by having a copy of the database at each exchange would be considerable.
What is needed therefore, and an object of the present invention, is an efficient and economical way of facilitating relocation of a subscriber from one exchange to another within a telecommunications network without changing the subscriber's directory telephone number. Such a relocation-accommodating system must be compatible with ISDN (integrated services digital network) services, especially those based on ETSI/CCITT standards.